On rare occasions chickens don't cross the road; they get stuck right in the middle, between male and female. A new study of these odd birds sheds light on the developmental processes that determine sex.

Contrary to an old view of sexual development, Michael Clinton and his colleagues at the Roslin Institute in Edinburgh say in the March 11 Nature, individual chicken cells can maintain their own strong male or female identities during development instead of being directed by hormones.

Clinton says his research group ended up considering hormones and sexual identity in the course of studying three peculiar chickens donated to the Roslin Institute. Each bird looked like a rooster on one side, with a long wattle jiggling under its chin, robust legs and bulging muscles. The other half of the same bird -- the right side on two birds and the left on the third -- had the darker plumage, reduced wattle and dainty ankles of a hen.

Such male-female mashups, called gynandromorphs, have turned up spontaneously in zebra finches, pigeons and parrots as well as in other kinds of animals, Clinton says. These cases challenge the traditional view that genetics takes a back seat to hormonal signals in guiding vertebrate sexual differentiation.

"The prevailing view of vertebrate sexual development is that the gonads form, then release hormones that masculinize or feminize the embryo," says another developmental biologist, Craig Smith of the University of Melbourne and Murdoch Childrens Research Institute in Parkville, Australia.

Studies in marsupials, mice and several birds have been chipping away at this dogma in recent years.

When Roslin's Derek McBride analyzed the tissues of gynandromorphic chickens, he found genetically male and female cells scattered all over their bodies, with one side consisting predominantly of genetically male cells while the other half had a female majority.

"Although gynandromorphs have been reported previously, they have not been analyzed at this level of detail," comments developmental geneticist Blanche Capel of Duke University in Durham, N.C.

Because cells in the same body experienced the same circulating hormones but retained different traits, the Roslin researchers hypothesized that a chicken cell's genetic identity trumps hormonal instructions.

Subsequent experiments supported the idea of strong, cell-by-cell sex identity. When the researchers transplanted tissues from genetically female embryos into what would become the gonads of genetically male ones and vice versa, the transplanted cells didn't start expressing opposite-sex characteristics.

In combination with other recent papers, says UCLA geneticist Art Arnold, the new study calls for fresh thinking about sex determination, and not just in birds. "The old hormone-only theory is no longer viable, for birds or mammals," he says.